Webster  Family  Library  of  Veterinary  Medicine 
Cummings  School  of  Veterinary  Medicine  at 
Tufts  University 
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North  Grafton,  MA  01 536 


1 


J,  I  /  /I  IBRARIES 


Notes  on  the  Osteology  of  the 
White  River  Horses. 


By  Marcus  S.  Farr. 


READ  BEFORE  THE  AMERICAN  PHILOSOPHICAL  SOCIETY. 

MAY  15,  1896. 


Beprinted,  Sept.  7*  1896)  from  the  Froo.  Amer.  FhiloBi  Soc,  Vol.  zxzv. 


Webb  LCI  rdjiijiy  uurary  oi  veiennai'y  Medicine 
Cummings  School  of  Veterinary  Medicine  at 
Tufts  University 
200  Westboro  Road 
North  Grafton,  MA  01 536 


TUFTS   UNIVERSITY    LIBRARIES 


3  9090  013  414  053 


1896.] 


[Farr. 


147 

Notes  on  the  Osteolocjy  of  the  White  River  Horses^ 

By  Marcus  S.  Farr. 

{Read  before  the  American  Philosophical  Society,  May  15,  ISOG.) 

MESOHIPPUS. 

Although  nearly  half  a  century  has  elapsed  since  3feso7iipptis  hairdi 
was  first  described  by  Leidy,*  our  knowledge  of  its  osteology  has 
remained  comparatively  incomplete,  all  the  known  material  being  lim- 
ited to  foot  bones  and  more  or  less  complete  skulls.  Most  all  of  the 
skeletons  that  were  found  were  badly  broken  up  and  only  the  larger 
and  more  perfect  bones  were  saved.  Modern  methods  of  collecting, 
essentially  those  introduced  by  Mr.  J.  B.  Hatclier,f  have  revolutionized 
all  this  and  now  even  the  most  delicate  bones,  though  badly  broken  up, 
are  preserved  as  easily  as  the  large  bones  were  before  collecting  was 
done  ill  a  scientific  manner. 

Fortunate  discoveries  of  more  complete  skeletons  during  the  last  three 
years  have  given  us  very  much  better  material  and  now  enable  us  to 
supplement  the  accounts  of  M.  bairdi  that  have  already  been  given,  to 
add  many  new  points  on  the  osteology  of  the  species  and  to  oflFer  a 
restoration  which  is  an  improvement  on  those  heretofore  offered. 

Several  species  of  Mesoliippus  have  already  been  made  on  material 
from  Nebraska,  Dakota  and  Colorado.  These  have  either  been  founded 
on  a  few  teeth  presenting  peculiarities  or  on  foot  bones  not  associated 
with  teeth.  These  species  have  not  been  generally  accepted,  and  the 
founding  of  species  on  such  limited  material  especially  in  such  a  genus 
as  Mesoliippus  which  presents  such  a  marked  degree  of  individual  varia- 
tion does  not  seem  justifiable  and  merely  burdens  science  with  useless 
synonyms.  I  have  not  seen  the  types  upon  which  the  various  species, 
M.  exoletum,X  M.  agrestis,%  M.  cuneatus,\\  M.  celer,^  etc.,  have  been  estab- 
lished, but  from  the  study  of  the  individual  variations  in  the  many** 
specimens  of  M.  bairdi  studied  by  the  writer  it  seems  very  evident  that 
the  species  are  not  well  grounded  and  that  the  peculiarities  may  be 
accounted  for  by  the  factor  already  mentioned. 

The  discoveiy  of  the  Protoceras  beds  and  their  recognition  as  a  dis- 
tinct subdivision  of  the  White  River  formationsff  marks  a  stage  in  the 
development  of  the  palaeontology  of  this  epoch. 

*  Leidy  first  described  this  species  as  Palseotherium  bairdi,  Proc.  Acad.  Nat.  Sci.,  18f  0, 
p.  122. 

t  Curator  of  Vertebrate  Palfeontologj-  in  the  College  of  New  Jersey. 

J  Cope,  U.  S.  Geol.  Survey  of  the  Territories,  1873. 

§  Leidy,  Rept.  U.  S.  Geo!.  Sur.  Terrs.  (4to),  i,  p.  251,  PL  vii. 

II  Cope,  Palxontl.  Bull.,  No.  10,  p.  7,  August  20,  1S73. 

1[  Marsh,  Am.  Jour.  Sci.  and  Arts,  1874,  p.  251. 

**  Remains  of  nearly  on^  hundred  individuals  have  been  studied  by  the  writer. 

tt  Wortman,  On  the  Divisions  oj  the  White  River,  Ball.  Am.  Mus.  Nat.  Hist.,  Vol.  v,  pp.  95- 
106. 


Farr.]  ^^"  [May  15, 

The  fauna  of  the  Protoceras  beds  is  unique  in  many  ways,  especially 
in  the  number  of  new  and  bizarre  forms  that  come  in,  some  evidently 
by  migration,  while  others  are  the  direct  descendants  of  the  species  of 
the  underlying  Oreodon  beds. 

These  strata  are  interesting,  as  they  form  a  transition  to  the  later  John 
Day  beds,  their  fauna  being  intermediate  between  the  latter  and  that  of 
the  Oreodon  beds. 

A  new  species  of  horse  has  been  found  in  this  formation  which  helps 
very  greatly  in  explaining  the  individual  variations  of  M.  bairdi,  as 
many  of  these  are  seen  to  be  attempts  in  the  direction  of  M.  intermedium, 
which  is  undoubtedly  the  direct  descendant  of  the  former.  Besides 
these  two  species  which  are  seen  to  stand  in  the  direct  relation  of  ances- 
tor and  descendant  there  is  another  species,  M.  copei,  which  occurs  first 
in  the  strata  of  the  Oreodon  beds  and  is  represented  in  the  Protoceras 
beds  by  larger  individuals. 

Geological  succession  of  the  species  : 
Protoceras  beds  :  M.  bairdi,  M.  copei,  M.  intermedius. 
Oreodon  beds  :  M.  bairdi,  M.  copei. 
Titanotherium  beds  :  M.  bairdi. 

The  genus  Mesohippus  occurs  then  in  all  the  different  horizons  of  the 
"White  River  beds.  In  the  Titanotherium  beds  it  is  usually  represented 
only  by  fragmentary  remains,  which,  however,  are  unmistakably  those 
of  31.  bairdi. 

The  Oreodon  beds  have  yielded  most  of  the  best  material.  Through 
the  whole  extent  of  the  fossiliferous  strata  of  these  beds,  a  vertical  thick- 
ness of  at  least  one  hundred  and  eighty  feet,  remains  of  M.  bairdi  are 
fairly  abundant.  However,  the  remains  are  not  well  preserved,  groups 
of  teeth  and  the  larger  limb  bones  are  common,  while  well-preserved 
portions  of  the  skeleton  are  rare — a  perfect  skull  has  never  yet  been 
found.  Beside  M.  bairdi  we  get  in  the  upper  Oreodon  beds  a  new  spe 
cies  which  has  been  described  as  31.  copei.*  The  Protoceras  beds  have 
yielded  only  fragmentary  remams  of  3f.  bairdi.  This  species  does  not 
represent  the  main  line  of  descent  during  this  epoch,  but  it  is  here  taken 
up  by  3f.  intermedius  while  the  former  still  persists  as  a  side  line.  We 
also  get  31.  copei,  which  continues  on  from  the  Oreodon  beds  and  is  now 
represented  by  larger  individuals. 

Of  31.  bairdi  newr}y  the  entire  skeleton  is  represented  by  material  in 
the  Princeton  collection. 

The  skull  has  been  quite  fully  described  by  Leidy,f  and  the  skeleton 
has  been  the  subject  of  an  exhaustive  paper  by  Prof.  Scott, |  but  when 
this  paper  was  written  the  entire  skeleton  had  not  yet  been  found  and 
the  incisor  teeth  of  upper  series  are  the  result  of  explorations  of  the 
summers  of  1894  and  1895,  so  some  points  in  the  description  will  be  new, 

*Osborn  ai)cl  Wortmai),  /'«?/.  Aw.  Miis.,  Vol.  vii,  pp.  35G-3.5''. 

t  Tlie  Extinrt  Mammalian  Funna  of  Dakota  and  Nebraska,  Philadelphia,  1809. 

XJourii.  of  Morvhvloyy,  Vol.  v,  No.  3,  December,  1891. 


1896.]  1 4  J  [Farr. 

Moreover  the  description  of  this  species  de  novo  is  justifiable  because  we 
wish  to  trace  the  steps  iu  the  evolutioa  of  the  horse  as  they  can  be  fol- 
lowed in  the  horizons  of  the  White  River  strata  and  must  therefore  have 
a  description  of  one  species  as  a  standard  for  comparison. 

It  is  the  purpose  of  this  paper  to  add  some  new  points  on  the  osteology 
of  M.  baircU  and  to  give  a  new,  more  accurate  and  more  complete  restora- 
tion ;  to  give  a  short  description  of  3/.  intermcdius  and  M.  cupei,  and  to 
show  their  relation  to  each  other  and  to  M.  hairdi. 

I  must  acknowledge  my  very  great  indebtedness  to  Prof.  Scott,  who 
has  given  me  so  much  assistance  in  the  way  of  suggestion  and  criticism 
and  whose  kindly  interest  in  my  work  has  ever  been  an  inspiration  dur- 
ing my  three  years  of  graduate  study  in  Princeton.  To  Mr.  J.  B. 
Hatcher  I  am  also  very  much  indebted  for  free  access  to  collections  and 
for  kindl}'  criticism  and  help  and  for  much  information  on  White  River 
mammals. 

I  must  also  extend  my  thanks  to  Prof.  H.  F.  Osborn  and  Dr.  J.  L. 
Wortman,  of  the  American  Museum,  for  permission  to  study  some  of 
their  very  beautiful  material  ;  also  to  the  latter  for  valuable  suggestions. 

The  drawings  are  by  Mr.  R.  Weber,  and  add  materially  to  the  value  of 
the  paper. 

The  Dentition 

The  dental  formula  is  I.  |,  C.  \,  Pm.  f,  M.  f.  The  dentition  is  thus 
seen  to  be  unreduced,  and  the  specialization  or  modernization  consists  in 
the  complexity  of  the  last  three  premolars  which  are  molariform  and 
Pm.  2  is  beginning  to  assume  the  elongate  character,  so  marked  in  the 
living  horse  by  the  elongation  of  the  anterior  part  of  the  external  half 
of  the  tooth.  The  characters  of  the  permanent  teeth  have  already  been 
described  by  Leidy,*  Osborn, f  and  Scott,:}:  but  very  little  has  been 
written  concerning  the  milk  dentition  and  the  superior  incisors  have 
only  very  recently  been  found.  Only  two  skulls  are  known  bearing  the 
upper  incisors,  nearly  all  the  skulls  that  are  discovered  having  the  end 
of  the  very  narrow  snout  broken  oft". 

The  inferior  canine  is  the  smallest  of  all  the  teeth  ;  it  is  suberect  and 
conical,  and  there  is  a  wide  diastema  between  it  and  Pm.  1.  The  lower 
incisors  are  spatulate  or  chisel  shaped  and  do  not  show  any  indication  of 
a  depression  or  pit.  Thej'  have  sharp  cutting  edges,  and  their  inner  sur- 
faces are  strongly  concave.  The  first  incisor  is  the  longest  {i.  e.,  highest 
above  alveolar  border)  and  also  the  widest  of  the  incisor  series.  I.  2  is 
smaller  than  I.  1,  while  I.  3  is  the  smallest  of  the  incisor  series.  There 
is  thus  a  decrease  in  size  and  length  of  incisors  outwardly  towards  the 
canine.     The  six  incisors  form  an  unbroken  row. 

*  Ancient  Fauna  of  Ntbmska,  pp.  70,  71 ;  Extinct  Mam.  Fauna  of  Dak.  and  Neb.,  pp.  3r5- 
309,  1869. 
t  Bnl!.  of  Mus.  of  Covip.  Zoij!.,  Vol.  xvi,  pp.  88,  89. 
f'Osrteologyof  Mesohippusand  Leptomeryx,"  Journ.  of  Morph.,  Vol.  v.  No.  3,  pp.  303-305. 


Farr.l  loO  [May  15, 

The  anterior  border  of  the  mandible  is  rounded  and  tlie  teeth  are  ar- 
ranged in  the  segment  of  a  circle. 

The  fourth  lower  premolar  is  wider  transversely  than  any  of  the  other 
teeth,  while  the  posterior  half  of  Pm.  3  is  wider  proportionately  than  any 
of  the  remaining  teeth,  with  the  exception  of  the  former.  Pm.  4  has  a 
massiveness  not  seen  in  the  other  lower  teeth.  Sometimes  this  is  so 
marked  that  if  the  teeth  were  not  found  together  they  would  in  all 
probability  be  attributed  to  a  larger  individual.  It  had  long  been  sup- 
l)osed  that  the  superior  incisors  were  not  pitted.  Prof.  Scott  *  has  sepa- 
rated Mesohippus  from  Miohippus  on  the  character  of  the  upper  incisors. 
A  skull  in  the  Princeton  collection  shows  the  upper  incisors  which  seem 
to  be  pitted,  but  as  they  are  so  much  worn  a  determination  of  their 
character  is  not  possible.  Osborn  and  Wortman  f  have  just  described 
these  teeth  and  through  the  kindness  of  these  gentlemen  I  have  been 
permitted  to  examine  this  beautifully  preserved  skull.  The  two  outer 
pairs  of  incisors  show  a  distinct  invagination,  which  is  not,  however, 
present  on  I.  1.  Upper  Pm.  1  is  a  small  single-coned  tooth,  which  has 
two  distinct  roots.  The  cingulum  is  well-developed  on  the  inner  side, 
enclosing  a  deep  pocket.  Anteriorly  there  is  a  tiny  accessory  conule. 
The  corresponding  tooth  of  the  lower  jaw  is  very  small  and  inserted  by 
only  a  single  fang.  Pm.  4  of  the  upper  series  is  wider  transversely  than 
any  of  the  other  teeth. 

SUCCE.SSION    OP     THE    TeETH. 

From  all  that  can  be  observed  the  three  large  deciduous  molars  first 
appear  simultaneously  in  both  jaws.  The  next  tooth  to  appear  is  that 
which  represents  Pm.  1  of  the  permanent  set.  Nothing  is  known  as  to 
the  time  of  appearance  of  the  incisors  and  canines,  but  judging  from 
analogy  we  may  presume  that  they  appear  as  early  as  the  milk  molars. 
A  mandible  of  Mesohippus  (No.  11107),  with  milk  dentition  and  M.  1 
of  the  permanent  set,  shows  alveoli  for  the  three  incisors  and  canine. 
The  next  tooth  to  appear  (after  persistent  Pm.  1)  is  M.  1,  which  is  suc- 
ceeded by  M.  2.  Next  the  temporary  molars  are  replaced  by  the  perma- 
nent premolars. 

In  the  upper  jaw  these  are  replaced  in  the  following  order:  Pm.  4,  Pm.  3, 
Pm.  2  (Pm.  1  persisting  in  both  jaws).  One  specimen  shows  Pm.  4  almost 
ready  to  erupt,  while  Pm.  3  is  very  much  smaller  and  the  germ  of  Pm.  2  is 
very  feebly  developed.  The  mode  of  succession  in  the  lower  jaw  seems 
to  follow  the  same  order.  In  specimen  No.  10995,  M.  1  and  M.  2  have  ap- 
peared, and  the  germs  of  the  permanent  teeth  are  seen  bj^  picking  away 
the  bone  and  exposing  roots  of  teeth,  where  the  germ  of  Pm.  4  is  seen  to 
be  better  developed  than  that  of  Pm.  3.     This  also  accords  -with  the  rate 

*  Trans.  Amer.  Phil.  Soc,  1883,  p.  79.  lu  the  light  of  present  knowledge  it  seems  best 
to  abandon  the  genus  Miohippus  and'  to  make  the  genus  Mesohippus  include  the  John 
Day  equines  as  well  as  the  White  River  forms. 

f  Bult.  Am.  Mas.  Nat.  Hist.,  Vol.  vii,  p.  353. 


1896.] 


151 


[Farr. 


of  wear  of  teetli,  as  Pm.  4  is  usually  more  ■worn  by  attrition  than  Pm.  B. 
After  the  deciduous  teeth  are  replaced  by  those  of  the  permanent  set, 
M.  8  appears  in  both  jaws. 

It  is  not  possible  to  tell  from  available  material  whether  the  incisors 
and  canines  are  replaced  or  are  persistent.  In  the  later  horse  from  the 
Equus  beds,  the  incisors  were  certainly  replaced,  and  the  germ  of  canine 
is  seen  piercing  the  jaw.  The  foramen,  through  which  it  is  growing,  is 
large,  but  it  is  not  possible  to  determine  whether  it  had  a  predecessor  or 
represents  a  permanent  canine  which  does  not  appear  until  the  other 
teeth  are  developed.  Chauveau*  makes  the  statement  that  the  canine 
persists  and  is  not  replaced  in  the  horse.  However,  judging  from 
analogy,  we  are  quite  safe  in  presuming  that  in  M.  huirdi  both  the  in- 
cisors and  canines  had  predecessors  in  the  milk  series. 


The  Milk  Dentition. 

The  temporary  dentition  may  be  given  in  the  following  formula  : 
I.  f,    C.  \,  D.  f.     See  Fig.  1. 

The  tooth  which  represents  Pm.  1  of  the  adult  skull  is  not  a  true  milk 
tooth,  as  it  does  not  appear  until  the  other  teeth  of  the  milk  set  are  full}^ 
developed,  and  is  not  replaced  as  are  the  teeth  of  the  temporary  series. 
It  may  be  considered  a  persistent 
milk  tooth,  as  it  has  no  predeces- 
sor,  and  then   the  dental   formula 
will  be  as  given  above.     If  consid- 
ered one  of  the  permanent  set,  as 
there  are  ample  reasons  for  doing, 
the  molar  formula  will  be  :  D.f. 

The  ditferences  between  the  de- 
ciduous teeth  and  those  of  the  per- 
manent set  are  not  due  to  any  ad- 
dition or  reduction  in  the  number 
of  elements  entering  into  the  forma- 
tion of  the  teeth,  but  are  due  to  the 
difl'erence  in  the  relative  develop- 
ment of  the  elements  in  the  two 
sets.  The  differences  can  best  be  described  by  instituting  a  comparison 
between  the  two  sets,  and  to  do  this  it  will  be  best  to  describe  those  of 
the  permanent  set  and  then  show  how  the  deciduous  molars  diti'er  from 
them.  The  last  two  of  the  temporary  set  ditfer  only  in  minute  detail 
from  the  corresponding  teeth  of  the  later  set,  but  there  is  a  fundamental 
difference  between  Pm.  2  of  the  permanent  set  and  its  predecessor  in  the 
milk  series. 

All  of  the  premolar  teeth,  with  the  single  exception  of  Pm.  1,  are 
molariform.     Pm.  3  of  both  jaws   presents   some  points  of  difference 


Milk  Molars  of  M.  bairdi,  \. 
a,  superior  series. 
b,  inferior  series. 


*  Comparative  Anatomy  of  the  Domesticated  Animals. 


Parr.]  1^2  [May  15, 

from  the  other  teeth,  while  the  simple  character  of  Pm.  1  has  already 
been  sufficiently  commented  upon.  The  last  lower  molar,  as  in  so 
many  forms,  diflers  from  the  others  in  the  presence  of  an  additional 
less  well-developed  lobe  situated  posteriorly.  The  lower  molars  and 
Puis.  3  and  4  have  oblono-,  quadrate  crowns,  with  an  outer  pair  of  fore 
and  aft  principal  lobes,  and  an  inner  pair  of  secondary  lobes  connate 
with  them.  "The  principal  lobes  of  the  crown  are  slightly  oblique  in 
their  relative  position,  angularly  convex  and  sloping  externally,  con- 
cavely  excavated  internally  and  are  acutely  crescentoid  at  their  summit. 
Of  the  inner  secondary  lobes,  the  anterior  is  much  the  larger,  and  is 
pyramidal  in  form  with  a  twin  pointed  summit."  This  character  is 
observable  only  in  teeth  that  are  not  worn  excessively  and  disappears  as 
the  summits  of  the  crown  are  worn  off  in  mastication.  "The  antero- 
internal  cusp  springs  from  the  crown  at  the  conjunction  of  the  principal 
lobes  and  is  continuous  with  their  contiguous  crowns.  The  posterior  of 
the  secondary  lobes  is  conical  and  springs  from  the  crown  in  conjunc- 
tion with  the  back  horn  of  the  posterior  principal  lobe.  The  front  horn 
of  the  anterior  principal  lobe  curves  inward,  downward  and  backward 
to  the  base  internally  of  the  anterior  secondary  lobe.  A  basal  ridge  (or 
cingulum)  nearly  continuous  bounds  the  crowns  of  the  lower  molars 
externally.  Posteriorly  it  rises  inward  and  terminates  in  a  tubercle 
springing  from  the  conjunction  of  the  two  posterior  lobes."  Pm.  2 
deserves  a  slight  mention  in  passing.  In  the  lower  jaw  the  posterior 
half  of  this  tooth  is  an  exact  copy  of  the  corresponding  part  of  any  of 
the  succeeding  premolars  or  molars.  One  half  of  the  antero-interual 
lobe  is  present  as  usual,  but  this  alone  forms  all  of  what  corresponds  to 
this  lobe  in  the  succeeding  teeth.  Anterior  to  this  and  externally  there 
is  another  lobe  more  nearly  median  in  position.  This  is  connected  with 
the  former  by  a  ridge  and  the  two  together  form  a  lobe  which  is  very  dif- 
ferent from  any  of  the  others.  Anterior  to  this  and  connate  with  it  is  a 
small  lobe  on  the  internal  surface  of  the  tooth.  The  deciduous  tooth 
differs  from  the  permanent  one  in  that  in  the  former  the  two  anterior 
lobes  are  more  distinct  from  each  other  and  from  the  other  lobes,  so  that 
we  seem  to  have  five  lobes  in  this  tooth.  Again  in  the  earlier  set  this 
tooth  has  a  greater  antero-posterior  extent  than  any  of  the  other  teeth, 
almost  equaling  in  length  M.  3  of  the  permanent  set,  which  has  the 
additional  lobe.  In  the  permanent  set  Pm.  2  is  even  shorter  antero-pos- 
teriorly  than  the  succeeding  tooth  in  the  premolar  series.  In  the  milk 
set  D.  4  has  the  posterior  half  narrower  than  the  anterior  half.  In  D.  3 
both  halves  of  tooth  are  of  approximately  the  same  width,  while,  in  the 
permanent  premolar  series  the  posterior  half  of  the  tooth  is  always  the 
wider,  while  in  the  molar  series  the  reverse  condition  obtains.  The  cin- 
gulum is  not  so  well  developed  on  the  deciduous  molars  as  on  the  corre- 
sponding teeth  of  the  permanent  set.  It  is  not  developed  on  the  external 
surface  of  the  posterior  lobe  as  in  the  permanent  tooth,  but  is  present  on 
the  posterior  border  of  tooth  where  it  ends  in  a  tubercle.     The  cingulum 


1896.]  ^^^  [Farr. 

is  well  developed  on  the  antero-external  lobe  of  Ds.  3  and  4,  even  better 
than  on  the  corresponding  permanent  tooth.  It  has  lately  been  called 
to  my  attention  that  the  cingnlum  varies  in  the  individual  with  the 
nourishment,  well-nourished  individuals  having  it  better  developed  than 
those  poorly  nourished,  but  the  recurrence  in  many  individuals  of  the 
character  as  given  above  precludes  the  possibility  of  its  being  an  indi- 
vidual variation.  The  antero-internal  cusp  is  wider  antero-posteriorly 
in  the  temporary  teeth  than  in  the  permanent  set  and  the  bifid  character 
of  this  cusp  is  more  marked  in  the  former.  All  the  lower  milk  teeth  are 
narrower  and  longer  antero-posteriorly  than  the  permanent  teeth.  Both 
the  upper  and  lower  molars  of  the  deciduous  set  are  of  not  nearly  so 
great  vertical  length  as  those  of  the  later  series. 

The  Upper  Molars. 

Premolars  2,  3  and  4  are  molariform  and  Pm.  2  is  beginning  to  assume 
the  elongate  character  which  is  so  much  emphasized  in  the  living  horse. 
The  six  molars  (i.  e.,  molars  and  molariform  premolars)  are  nearly  alike 
in  size  and  form.  "They  have  square  crowns,  wider  transversely  than 
broad  antero-posteriorly  and  both  these  measurements  greatly  exceed 
the  length.  The  crowns  consist  of  three  pairs  of  lobes — an  outer  and 
an  inner  pair  of  principal  lobes  and  a  much  smaller  pair  situated 
between  them,  the  secondary  or  accessory  lobes.  The  outer  lobes  are 
demi-conoidal  and  form  at  their  junction  a  narrow  buttress  externally. 
A  stronger  buttress  bounds  the  fore  part  of  the  anterior  of  the  two 
lobes.  A  tendency  to  the  development  of  a  buttress  is  seen  also  at  the 
back  part  of  the  posterior  of  these  lobes.  The  buttresses  expand  and 
are  conjoined  at  the  bottom  of  the  crown,  forming  together  a  pair  of 
arches  bounding  the  external  surfaces  of  the  outer  lobes.  These  sur- 
taces  are  nearly  flat  and  are  divided  by  a  conspicuous  median  ridge. 
The  inner  surfaces  of  the  outer  lobes  are  prominently  or  almost  angu- 
larly convex.  The  inner  lobes  of  the  crown  are  simply  conical,  wider 
ti'ansversely  than  fore  and  aft  and  with  the  anterior  slightly  larger  than 
the  posterior.  The  median  lobes  are  not  more  than  half  the  size  of  the 
principal  ones  and  appear  as  prominent  folds  curving  outwardlj'^  from 
the  inner  lobes  to  the  anterior  face  of  the  outer  lobes.  Elements  of  a 
basal  ridge  exist  at  the  fore  and  aft  parts  of  the  crown  and  at  the  outlet 
of  the  valley  separating  the  inner  lobes.  In  the  interval  posteriorly 
between  the  back  inner  and  outer  lobes  there  exists  a  tubercle  which  in 
association  with  the  contiguous  portions  of  the  basal  ridge  assumes  the 
dignity  of  a  sublobe."  In  Pm.  1  the  anterior  buttress  is  more  distinct 
or  separate  than  in  the  other  molars,  though  it  is  not  so  large.  The 
anterior  of  the  median  cusps  is  larger  than  the  posterior,  except  in  Pm.  2, 
which  is  peculiar  in  this  as  in  so  many  other  respects. 

The  teeth  of  the  temporary  set  present  the  following  differences  from 
those  of  the  permanent  set  described  above  : 

1.  The  cusp  situated  between  the  outer  and  inner  posterior  lobes,  the 


Farr.]  154  [May  15, 

so-called  hypostyle,  is  less  well  developed  in  the  deciduous  molars  than 
in  those  of  the  permanent  set. 

2.  D.  2  is  much  larger,  more  elongate  antero-posteriorly,  more  com- 
plex, the  antero-external  buttress  being  much  larger  and  more  distinct 
in  the  earlier  set.  It  is  so  large  that  it  might  almost  be  considered  a  fifth 
principal  lobe. 

3.  The  median  accessory  lobes  (5  and  6)  are  more  conical  than  in  the 
permanent  set,  where  they  are  somewhat  appressed.  These  lobes  in  the 
early  set  are  separated  by  a  distinct  notch  from  the  internal  lobes. 

4.  The  transverse  ridges  are  more  nearly  confluent  with  the  outer  wall 
of  tooth  in  most  of  specimens  in  the  temporary  set.  There  is,  however, 
great  individual  variation  in  regard  to  this  character. 

5.  In  the  adult  skull  all  the  molars  and  molariform  premolars  are 
much  wider  transversely  than  antero-posteriorly.  The  deciduous  teeth 
are  more  nearly  square,  the  two  diameters  being  subequal. 

6.  The  buttress  on  the  antero-exteriial  lobe  of  tooth,  the  parastyle,  is 
better  developed  in  the  milk  set. 

7.  D.  2  is  the  longest  tooth  of  the  milk  series  and  is  beginning  to 
assume  the  elongate  character  of  this  tooth  in  the  modern  horse,  while 
the  corresponding  tooth  of  the  permanent  set  is  smaller  than  any  of 
the  other  molar  teeth. 

8.  All  the  temporary  teeth  are  shorter  in  vertical  height  than  those  of 
the  permanent  set. 

The  Vertebral  Column. 

The  cervical  and  dorsal  vertebrae  have  already  been  minutely  de- 
scribed. The  lumbar  vertebrae  are  almost  certainly  five  in  number. 
The  centra  are  large  and  are  reniform  in  shape,  being  wide  transversely 
and  not  having  the  more  nearly  circular  outline  of  the  median  dorsal 
vertebrae.  All  of  the  lumbars,  with  the  exception  of  the  last,  have  their 
centra  strongly  keeled.  The  centra  are  moderately  opisthoccelous.  The 
interlocking  character  of  the  vertebra;  through  the  zygapophyses  is 
marked.  The  neural  spines  are  long,  transversely  compressed  and 
narrow  and  have  considerable  antero-posterior  extent.  They  are  all 
directed  forward  at  an  angle.  The  transverse  processes  are  well  devel- 
oped and  widely  expanded.  The  intervertebral  foramina  perforate  the 
bases  of  the  neural  arches,  and  are  not  merely  notches  in  the  ends  of 
the  neural  arch  as  they  are  in  the  anterior  vertebrae  of  the  column.  The 
last  two  lumbar  vertebrae  have  their  transverse  processes  expanded 
almost  as  widely  as  those  of  the  first  sacral  itself,  and  the  transverse 
processes  of  the  fourth  lumbar  abut  against  those  of  the  fifth,  while  the 
latter  bears  on  the  posterior  surface  of  the  transverse  processes  deep 
concavities  for  the  corresponding  surfaces  of  the  anterior  end  of  sacrum. 
An  analogous  condition  is  seen  in  Equus,  and  in  old  individuals  the  last 
two  lumbars  are  very  frequently  immovably  coiissified.  The  last  lumbar 
lias  the  spine  more  nearly  erect  than  that  of  the  penultimate  luml)ar 
vertebra. 


1896.] 


155 


[Farr. 


Fig. 


A  very  remarkable  cliaracter  of  the  lumbar  vertebrae  is  that  they  have 
spines  which  are  nearly,  if  not  quite,  as  high  as  those  of  the  anterior 
dorsal  region,  which  in  the  horse  are  so  much  elongated.  In  the  latter 
the  lumbars  have  spines  which  are  lower,  more  nearly  erect,  of  more 
considerable  antero-posterior  extent  proportionately  and  are  much  less 
compressed  transversely. 

The  Saceum. 

The  sacrum  of  M.  hairdi,  as  in  most  of  the  Ungulata,  consists  of  one 
broad  vertebra  joining  the  ilia,  followed  by  a  series  of  narrower  ones, 
gradual]}'  diminishing  in  width  anchylosed  to  it  behind.  Tliese  latter 
diminish  in  width  very  gradually.  In  living  Ungulates  the  number  of 
vertebra;  entering  into  the  formation  of  the  sacrum  varies  with  the  age 
of  the  individual  and  also  varies  in  individuals  of 
the  same  age. 

In  the  specimen  which  belongs  with  the  pelvis 
described  below  there  are  six  vertebrae.  This  is 
the  most  perfect  sacrum  of  M.  bairdi  yet  found, 
and  the  component  vertebrae  are  fortunately  well 
preserved  and  hardly  crushed  at  all  (see  Fig.  2 
and  Plate  xiii). 

The  first  or  true  sacral  vertebra  is  greatly  ex- 
panded transversely  and  bears  large  articular  sur- 
faces for  the  ilia. 

Anteriorly  there  are  large  convex  facets  which 
fit  into  the  corresponding  concavities  in  the  trans- 
A'erse  processes  of  the  last  lumbar  vertebra.  The 
first  sacral  has  a  low  and  comparatively  wide  cen- 
trum. The  spine  is  very  high,  very  much  com- 
pressed laterally,  as  are  all  the  spinous  processes  of 
the  vertebrae,  and  is  directed  strongly  forward,  while  in  the  modern 
horse  it  is  almost  vertical.  The  five  succeeding  vertebrae  have  trans- 
verse processes  which  are  not  so  widely  expanded,  the  centra  are  very 
much  depressed  and  the  neural  arches  are  low  and  gradually  decrease 
in  height  posteriorly.  This,  of  course,  conditions  the  size  of  the  neural 
canal,  which  in  this  region  is  very  much  attenuated.  The  expanded  trans- 
verse processes  of  the  contiguous  vertebrae  are  all  united,  so  that  they 
form  a  narrow  elongate  plate.  The  spine  of  the  second  sacral  is  gone,  but 
the  others  are  all  preserved.  That  of  the  third  is  almost  vertical,  while 
the  spines  of  the  three  posterior  sacrals  all  slope  backward  at  a  decided 
angle.  There  is  thus  a  very  abrupt  transition  in  the  direction  of  the  incli- 
nation of  the  spines  from  the  first  in  which  the  spine  projects  forward 
to  three  in  which  the  process  is  almost  vertical.  The  plate  formed  by 
the  anchylosis  of  the  centra  and  transverse  processes  of  the  vertebrae  is 
concave  interiorly  or  curves  downward  posterior  to  first  sacral.  The 
sacrum  presents  inferiorly  the    foramina    for  the  five  pairs  of   sacral 


Saceum  of  M.  b.\irdi. 
Inferior  view. 


Farr.]  J  OO  [May  15, 

nerves,  the  inferior  sacral  foramina,  while  above  we  also  find  laterally 
between  the  nem-al  arches  of  the  contiguous  vertebrte  the  five  pairs  of 
the  superior  sacral  foramina. 

Measurements  of  the  Sacrum.  mm. 

Length 116 

Extreme  width 64 

Width  third  sacral 23 

Width  fourth  do 21.5 

Width  fifth       do 20 

The  Caudals. 

The  few  caudal  vertebrae  preserved  are  sufficient  to  give  us  a  general 
idea  of  the  character  of  the  tail.  The  first  caudal  has  very  widely  ex- 
panded transverse  processes  similar  to  those  of  the  posterior  sacral  re- 
gion ;  the  centrum  is  oval  and  the  neural  arches  arise  at  a  very  great 
angle  enclosing  a  high  and  very  narrow  neural  canal.  The  transverse 
processes  are  of  considerable  antero-posterior  extent,  but  do  not  equal 
the  length  of  the  centrum  in  width  as  they  do  in  the  posterior  A^ertebrse 
of  the  sacral  region.  It  is  not  possible  to  determine  how  many  of  the 
caudal  vertebrae  had  complete  arches,  because  of  incomplete  material. 
In  Equus*  the  spine  of  the  neural  arch  is  bifid  in  the  second  caudal  and 
the  arches  are  incomplete  on  the  third.  The  transverse  processes  gradu- 
ally become  shorter,  the  neural  arches  more  rudimentary  and  are  finally 
lost,  and  all  we  have  is  a  cylinder  of  bone  with  very  rudimentary  pro- 
cesses which  gradually  diminish  in  size.  Among  the  caudals  preserved 
is  one  of  these  last,  in  which  all  the  processes  are  very  feebly  developed. 
All  the  vertebrae  of  the  tail  are  in  general  like  those  of  the  horse,  and  in 
them,  as  in  most  all  of  the  anatomical  features,  we  see  a  foreshadowing 
of  what  the  future  horse  is  going  to  be. 

The  Sternum. 

With  tlie  almost  complete  skeleton  figured  in  the  restoration  of  M. 
bairdi  in  Plate  xiii  are  preserved  tliree  segments  of  the  sternum.  These 
are  the  xiphisternum  and  two  segments  of  the  mesosternum.  The 
former  is  very  much  more  elongate  and  not  so  high  as  the  other  divisions 
of  the  sternum.  Anteriorly  it  is  about  twice  as  broad  as  high,  while 
posteriorly  it  is  A'ery  much  flattened.  The  free  border  is  thin  and 
rounded  with  irregular  surface,  showing  where  cartilage  was  attached. 
Laterally  the  body  of  this  segment  as  of  all  the  other  is  concave.  The 
superior  border  is  almost  plane,  while  the  inferior  is  slightly  concave,  or 
the  free  end  may  be  said  to  project  slightly  downward. 

The  next  segment  in  front  of  the  above  that  is  preserved  is  very  evi- 
dently the  penultimate  segment  of  the  mesosternum.  This  is  very  differ- 
ent in  shape  from  the  xiphisternum.     The  posterior  portion  is  wide  and 

*  No,  33P,  Princeton  Coll. 


1896.] 


157 


[Farr. 


low,  while  anteriorly  it  is  much  narrower  and  higher.  Both  superior  and 
inferior  surfaces  are  plane  and  the  sides  are  very  strongly  concave.  The 
third  segment  is  evidently  the  first  division  of  the  mesosternum,  and  is 
high  and  long  and  almost  trihedral  in  cross-section.  These  separate 
segments  of  the  sternum  are  not  coossified,  and  the  surfaces  for  the  articu- 
lation of  the  sternal  cartilages  of  ribs  are  not  well  shown.  From  the  por- 
tions of  sternum  described  above  we  are  safe  in  assuming  that  there  were 
at  least  six  segments  in  the  sternum  of  31.  bairdi. 

The  Scapula. 

The  nearly  complete  skeleton  from  which  the  restoration  given  here- 
with is  made  fortunately  has  the  scapula  very  well  preserved,  and  this 
reveals  quite  an  unexpected  character,  viz.,  the  presence  of  a  distinct 
acromion.  The  only  other  Perissodactyl  known  to  have  retained  this 
process  is  Pachynolophus  (Orohippus)  of  the  Bridger  Eocene.  Marsh  * 
has  described  it  in  this  genus  as  follows  : 
acromial  process,  which  is  com- 
pressed and  decurved  as  in  some 


•  The  scapula  has  a  prominent 
Fig.  3. 


Carnivora."  Mesohippus  is  the 
only  Perissodactyl  known  to  have 
retained  this  process  until  Oligo- 
cene  times,  and  it  has  thus  been 
retained  longer  by  the  horses  than 
by  any  otlier  family  of  this  order. 
It  is  possible  that  future  discoveries 
may  also  reveal  the  presence  of  a 
clavicle  in  Mesohippus,  as  it  has 
been  discovered  in  the  contem- 
porary Oreodon  culiertsoni,\  and  in 
the  latter  genus  it  persists  until 
Deep  River  times,  where  it  has 
been  found  by  Prof.  Scott  X  in  the 
form  which  he  has  called  Mesoreo- 
don.  The  possession  by  both 
Mesohippus  and  Pachynolophus  of 
this  process  would  seem  to  justify 
us  in  regarding  the  latter  as  the 
Bridger  ancestor  of  the  horse  line 
of  which  Mesohippus  is  the  White 
River  representative.  The  scapula 
is  wider  in  proportion  to  its  height 
than  that  of  Equus.  The  anterior 
margin  is  very  thin  and  strongly 

*Am.er.  Jour.  Sc.  and  Arts,  Series  3,  Vol  vii,  1874,  p.  247 

t  A  specimen  in  the  museum  of  tlie  University  of  Chicago  reveals  the  presence  of  the 
clavicle. 
I  Trans.  Amer.  Philos.  Soc,  Vol.  xvii,  p.  136. 


Scapula  of  M.  bairdi,  %. 

a,  from  outside. 

b,  from  behind. 

c,  from  below. 


Farr.]  lOO  []SIay  15, 

convex,  wliile  the  posterior  border  is  only  slightly  rounded  and  is  very 
much  thickened,  a  character  that  has  been  retained  by  the  Equidae,  Tylo- 
poda,  Pecora  and  Suina,  but  has  been  lost  in  the  Tapiridae  and  Rhinocer- 
otidfe.  The  spine  of  the  scapula  is  very  high  and  seems  to  extend  nearly  or 
quite  to  the  vertebral  border.  It  is  much  nearer  to  the  anterior  border  than 
the  posterior,  thus  making  the  prescapular  fossa  much  smaller  than  the 
postscapular. 

The  spine  becomes  gradually  more  prominent  towards  the  middle  por- 
tion, at  which  point  it  seems  to  have  been  highest  and  the  edge  was  here 
strongly  retroverted  as  in  Tapirus  and  Rhinoceros.  From  this  point  it 
decreases  in  height  towards  the  vertebral  border. 

The  acromion  is  styliform  in  shape,  is  compressed  antero-posteriorly 
and  extends  outward  and  downward,  but  does  not  quite  reach  the  level 
of  the  glenoid  cavity.  It  resembles  in  shape  that  of  the  camel  and 
llama,  but  dift'ers  from  these  in  that  they  are  more  slender,  more  nearly* 
perpendicular  and  extend  nearly  or  quite  to  the  level  of  the  glenoid 
cavity.  The  process  gradually  tapers  towards  the  free  end,  which  is 
somewhat  rounded.  The  neck  of  the  scapula  is  very  much  constricted 
and  is  comparatively  long.  The  glenoid  cavity  is  quite  deeply  ex- 
cavated, is  very  slightly  elongate  antero-posteriorly  and  has  a  well-de- 
fined rim. 

The  coracoid  process  is  strong,  curves  inwardly  and  is  slightly 
retroverted. 

Measurements  of  Scapula.  mm. 

1.  Extreme  length 136 

2.  Width  of  neck 18 

3.  Width  of  distal  end 32 

4.  Extreme  width 74 

5.  Width  at  highest  point  of  spine 74 

6.  Width  of  supra-spinous  fossa  here  25 

7.  Width  of  infra-spinous  fossa  here , 45 

Measurements  of  Scapula  of  Equus*  mm. 

1.  Extreme  length 414 

2.  Width  of  neck 73 

3.  Width  of  distal  end 107 

4.  Extreme  width 18(5 

5.  Width  at  highest  point  of  spine 14G 

6.  Width  of  supraspinous  fossa  here 48 

7.  Width  of  infra-spinous  fossa  here 93 

These  measurements  show  the  scapula  of  M.  bairdi  to  have  been  pro- 
portionately more  expanded  superiorly  than  that  of  the  horse  and  at  the 
same  time  the  neck  is  proportionately  more  contracted  than  in  the  latter. 

*No.  338,  Princeton  CoU. 


1898.] 


159 


[Farr. 


Fis.  4. 


Pelvis  of  M.  bairdi,  %. 


The  Pelvis  (No.  11376). 

The  pelvis  is  equine  in  all  its  cliaracters  and  very  much  like  that  of  the 
modern  horse  with  some  characteristic  points  of  difference.  The  speci- 
men described  below  is  the  first  pelvis  of  3fe8ohij)pus  bairdi  that  has  ever 
been  found  showing  all  the  characters,  being  almost  perfect.  See  Fig. 
4,  and  Plate  xiii.  It  was  discovered  by 
Mr.  J.  W.  Gidley  during  the  past  sum- 
mer in  the  lower  Oreodon  beds. 

The  most  striking  difference  between 
the  pelvis  of  M.  bairdi  and  that  of  tlie 
horse  is  that  the  former  is  narrower  in 
proportion  to  its  length  tlian  that  of 
Equus. 

The  great  breadth  of  the  pelvis  an- 
teriorly in  the  latter  is  owing  to  the  very 
great  lateral  expansion  of  the  ilia,  while 
in  the  earlier  genus  they  are  propor- 
tionately less  widely  expanded.  The  ilia 
directly  in  front  of  the  acetabulum  are 
slender  in  their  proportions  and  expand 
more  gradually  than  in  the  horse,  so  that 
they  are  longer  in  proportion  to  their 
width  than  in  the  latter.  The  bone  is 
widely  expanded  superiorly  and  the  angle  above  the  point  of  articulation 
of  the  ilium  with  the  sacrum  curves  upward  and  outw^ard,  and  the  free 
end  is  thickened  and  somewhat  rugose.  This  upward  and  outward  ex- 
l^ansion  of  angle  makes  the  external  border  of  superior  aspect  of  the 
ilium  concave.  The  crest  is  more  slender  and  elongate  comparatively 
than  in  Equus  and  is  strongly  everted.  The  border  of  the  ilium  between 
the  angle  and  the  crest  is  very  thin  and  strongly  concave.  The  whole 
anterior  expanded  portion  is  thin  except  along  the  outer  or  lower 
border.  The  posterior  border  of  the  angle  above  the  point  of  articula- 
tion of  the  sacrum  is  also  slightly  thickened.  The  sacral  border  of  the 
ilium  is  large  and  extends  high  above  the  articular  facet  for  the  sacral 
vertebrae  forming  the  angle.  The  ilia  as  well  as  the  long  axis  of  pelvis 
are  directed  downAvard  at  an  angle  from  the  vertebral  column.  The 
acetabulum  is  an  elongate  oval  in  shape  and  its  borders  are  elevated  and 
well-defined.  The  border  is  incomplete  below  owing  to  the  encroach- 
ment of  the  pit  for  the  ligameutum  teres  on  the  acetabular  fossa.  This 
is  less  emphasized,  however,  than  in  the  horse.  The  pit  for  the  liga- 
mentum  teres  is  quite  deep. 

The  ischium  is  straight  and  on  a  line  with  the  long  axis  of  the  ilium. 
The  bone  curves  outwardlj^  posteriorly,  but  does  not  curve  upward  as  in 
the  horse.  The  posterior  border  is  expanded  and  thickened  outwardly 
where  it  ends  in  a  stout  process,  the  tuberosity  of  the  ischium.  The  in- 
ternal border  posteriorly  is  deflected  towards  the  median  line  and  meets 


Farr.]  lt)U  [May  15, 

its  fellow  of  the  opposite  side  at  this  point  forming  part  of  the  symphysis. 
Above  the  acetabulum  the  border  of  bone  is  high  and  rounded,  but  is  not 
sharp  and  angular  as  in  the  horse.  The  obturator  foramen  in  the  pelvis 
of  the  latter  is  rounded  and  shorter  in  proportion  to  its  width  than  in 
M.  bairdi,  being  only  slightly  elongate,  while  in  the  species  under  con- 
sideration the  foramen  is  narrow  and  very  much  elongated,  the  length 
equaling  twice  the  breadth.  This  conditions  the  shape  of  the  posterior 
portion  of  ischium,  which  in  M.  bairdi  does  not  extend  far  back  of  the 
posterior  border  of  obturator  foramen,  while  in  Equus  the  ischium  forms 
a  large  expanded  plate  posterior  to  the  obturator  foramen. 

The  pubis  is  elongate,  flattened  from  above  downward  and  irregularly 
triangular  in  shape.  The  portion  of  pubis  nearest  the  acetabulum  is 
almost  round  in  cross-section,  while  in  the  horse  the  corresponding  por- 
tion, as  in  fact  the  entire  pubis,  is  very  much  more  flattened.  It  meets 
its  fellow  of  the  opposite  side  in  the  median  line  forming  the  anterior 
part  of  the  symphysis  with  the  bases  of  the  triangles  applied  together 
The  symphysis  is  formed  by  both  pubes  and  ischia  conjointly,  the  former 
constituting  the  anterior  and  larger  part  while  the  ischia  form  the  poste- 
rior part.  Fusion  of  the  pubes  is  so  complete  that  no  trace  of  a  suture 
remains,  while  the  ischia  are  not  anchylosed  together.  The  anterior 
part  of  the  symphysis  is  flattened  in  the  form  of  a  large  plate,  which 
bears  inferiorly  in  the  median  line  a  prominent  spine.  All  the  processes 
for  muscular  attachments  are  less  strong  and  rugose  than  in  the  horse. 
The  pelvic  foramen  (or  cavity)  is  longer  in  proportion  to  the  breadth  in 
M.  bairdi  than  in  the  horse,  being  a  little  longer  than  broad,  while  in 
the  latter  the  pelvic  outlet  is  broader  than  long.  In  Mesohippus  the 
length  (or  vertical  height)  is  about  65  mm.  and  the  breadth  00  mm., 
while  in  the  horse  the  reverse  condition  obtains  and  we  find  a  length 
of  only  174  mm.  as  compared  witli  a  width  of  199  mm.* 

Other  measurements  of  the  pelvis  are  as  follows  : 

MM. 

1.  Extreme  length 209 

2.  Length  of  acetabular  cavity 26 

3.  Length  of  symphysis 63 

4.  Extreme  width  of  ischia 74 

5.  Width  at  acetabulum 102 

6.  From  top  of  angle  to  outer  point  of  crest 89 

7.  From  anterior  border  of  acetabulum  to  point  midway 

between  angle  and  crest 74 

Restoration  op  M.  bairdi  (Pl.  xiii). 

In  1879,  Prof.  Marsh,f  in  giving  the  genealogy  of  the  horse,  brought 
out  the  fact  that  the  chief  modifications  through  which  the  horse  passes 
in  its  evolution  are  the  following  : 

1.   Progressive  increase  in  the  length  of  teeth  and  in  their  complexity, 

*  (6A  X  7/5  inches)  Cliauveau  loc.  cit.  1;Am.  Jour.  ScL,  Vol.  xvii,  p.  497. 


1896.]  i^i  [Farr. 

from  a  very  short-crowned  tooth  with  distinct  roots,  to  one  with  very 
long   crown  in  which  roots  are  not  formed  till  animal  becomes  adult. 

2.  The  gradual  lengthening  of  the  limb  bones  with  the  suppression  of 
the  lateral  digits  and  the  concentration  of  the  growth  force  in  metapo- 
dial  iii,  producing  ultimately  a  monodactyl  foot  from  a  pentadactyl 
ancestor. 

3.  The  continued  reduction  of  ulna  and  fibula  and  their  ultimate 
ooalescence  with  the  radius  and  tibia. 

4.  Gradual  increase  in  size  from  an  animal  not  larger  than  a  fox  up  to 
the  modern  horse. 

MesoJujjpus  bairdi  is  an  interesting  intermediate  stage  in  the  evolution 
of  the  horse  ;  though  primitive  in  many  respects,  it  had  already  made 
considerable  advance  over  its  Uinta  predecessor. 

The  restoration  here  given  is  made  from  a  nearly  perfect  skeleton 
which  enables  us  to  make  some  improvements  on  the  one  already  given,* 
which,  however,  was  as  good  as  could  be  made  with  the  material  then 
available. 

The  lumbar  vertebrae,  sacrum,  pelvis  and  a  few^  of  the  posterior  dor- 
sals are  from  another  individual  reduced  to  proportion.  Part  of  the 
skull  is  also  restored  from  another  specimen. 

Mesohippus  occupies  a  position  about  midway  in  the  line  of  descent  of 
the  horse  series.  It  presents  the  following  advances  over  its  Bridger 
predecessor,  Pachynolophus. 

1.  The  teeth  are  longer  (vertically)  and  more  complex,  the  interme- 
diate cusps  are  better  developed,  and  the  transverse  ridges  are  likewise 
better  developed  and  more  nearly  confluent  with  outer  wall  of  tooth. 

2.  The  lateral  metapodials  are  more  reduced  comparatively,  and  meta- 
podial  iii  is  much  larger.  In  the  Bridger  form  the  phalanges  of  the 
fifth  digit  are  present,  but  M.  bairdi  has  lost  these. 

3.  Both  the  ulna  and  fibula  are  more  reduced  than  in  the  earlier  form 

4.  In  M.  bairdi,  Pms.  2-4  are  molariform,  while  in  Pachynolophus 
Pm.  4  only  is  molariform  and  is  smaller  than  true  molars.  Epihippus. 
the  Uinta  representative  of  the  series,  has  Pms.  3  and  4  molariform,  and 
this  is  the  only  generic  distinction  between  the  Bridger  and  Uinta 
genera. 

The  orbit  is  commencing  to  retreat,  though  it  is  still  over  the  molars, 
the  anterior  border  being  directly  over  the  posterior  half  of  M.  1.  In  the 
horse  it  is  situated  posterior  to  molar  series,  and  we  can  trace  a  gradual 
transition  in  the  position  of  orbit  up  through  the  different  genera  from 
Mesohippus  to  Equus.  This  shifting  backward  of  the  orbit  brings 
?ibout  a  gradual  elongation  of  the  facial  region  of  the  skull.  The  alve- 
olar border  of  the  maxillaries  is  low,  this  of  course  being  associated- with 
low-crowned,  short-rooted  teeth. 

From  the  character  of  the  teeth  we  may  judge  of  the  life  habits  of 
the  animal.     The  teeth  of  the  modern  horse  have  very  long  crow^ns 

*Journ.  of  Morph.,  Vol.  v,  No.  ^,  p.  337. 


Farr.]  lo^  [May  15, 

(hypsodont),  grow  from  persistent  pulps  and  do  not  form  distinct  roots 
until  the  animal  is  quite  old,  not  until  a  length  of  crown  is  attained 
Avhich  under  normal  conditions  will  afford  sufficient  grinding  surface  for 
an  average  lifetime.  As  the  teeth  wear  oft'  by  attrition  the  loss  is 
replaced  by  growth,  and  growth  and  w'ear  proceed  pari-passu  until  the 
animal  becomes  adult. 

The  little  Mesohippus,  with  its  short-crowned  (brachyodout)  teeth, 
inserted  by  distinct  roots,  must  therefore  have  fed  on  succulent  plants 
that  grew  in  swampy,  marshy  land — as  if  subjected  to  wear  necessitated 
by  the  mastication  of  the  hard,  silicious  grasses  of  Miocene  times,  the 
teeth  would  soon  have  worn  out  entirely  and  the  animal  would  have 
succumbed  to  starvation.  In  most  of  the  specimens  found  the  teeth  are 
only  moderately  abraided. 

The  feet,  too,  being  tridactyl  are  adapted  to  progression  along  the 
oozy  shore  of  rivers  or  to  swampy,  marshy  ground  as  the  toes  would 
spread  and  thus  support  the  animal  in  the  mud,  while  the  monodactyl 
foot  of  the  horse  is  preeminently  adapted  for  rapid  locomotion  over  the 
grassy  plains.  This  would  seem  to  prove  that  the  life  habits  of  the  ani- 
mal have  changed  very  greatly  during  its  evolution.  Many  of  the 
White  River  animals  were  adapted  by  their  anatomical  structure  to  life 
in  swamps.  Some  wei'e  at  least  semi-aquatic  in  their  habits,  as  is  denoted 
by  the  position  of  the  posterior  nares,  which  in  some  forms  are  removed 
verj'  far  backward,  e.  g.,  Ancodus. 

The  skull  is  equine  in  its  characters,  but  is  still  quite  small  and  the 
facial  region  is  short.     The  orbit  is  not  enclosed  behind. 

The  neck  is  long,  and,  as  in  the  horse,  these  vertebrae  are  larger  than 
those  of  the  dorsal  region  of  the  column.  The  processes  are  not  so  mas- 
sive as  in  Equus,  but  are  quite  as  complex  and  are  very  well  developed. 
The  spines  of  the  dorsal  vertebrae  are  not  so  high  as  we  should  expect, 
and  very  evidently  M.  hairdi  did  not  have  any  great  elevation  of  the 
anterior  dorsal  region.  The  modern  horse  is  much  higher  at  the  withers 
tlian  at  the  haunches.  The  spines  of  the  lumbar  vertebrae  are  \qyj 
high  and  incline  forward  at  quite  an  angle.  There  is  a  very  abrupt 
transition  in  height  of  spines  from  the  first  sacral,  which  has  a  very 
high  spine  to  third  sacral,  which  has  a  very  much  lower  spine, 
though  it  is  still  much  compressed  laterally.  Six  vertebrae  take  part 
in  the  formation  of  the  sacrum.  The  centra  of  the  first  few  caudals 
are  flat  with  wide  transverse  processes,  but  these,  as  well  as  all  the  other 
processes,  gradually  become  suppressed  and  the  neural  arches  disappear 
so  that  the  lower  caudals  are  merely  cylinders  of  bone.  It  is  impossible 
to  determine  the  exact  number  of  vertebrae  taking  part  in  the  formation 
of  the  tail,  but  it  is  fair  to  imagine  that  it  had  one  at  least  as  long  pro 
portionately  as  the  horse. 

The  scapula  is  remarkable  for  the  persistence  of  the  acromion  process, 
in  which  character  it  is  unique  among  all  Perissodactyls,  with  the  excep- 
tion of  Pachynolophus  (Orohippus)  of  the  Bridger.     The  spine  is  better 


Proceedings  llmer,  Pliilos,  Soc, 


1896.]  ^^^  [Farr. 

developed,  the  bone  is  lower  and  broader,  the  neck  is  more  constricted 
proportionately  than  in  the  horse.  In  the  latter  the  anterior  border  of 
the  scapula  is  not  rounded  as  in  Mesohippus.  The  ulna  is  very  much 
reduced  in  M.  bairdi,  and  the  radius  is  enlarged  to  sustain  the  weight  of 
body.  The  ulna  is  distinct  from  the  radius  through  the  whole  of  its  ex- 
tent, the  two  bones  not  being  coossified  even  in  old  individuals.  Below 
the  proximal  half  the  bone  is  much  compressed  and  tapers  rapidly  to- 
ward the  distal  end.  This  gives  it  a  frail  character  so  that  it  is  almost 
always  broken  away  in  fossilization,  and  only  recently  have  specimens 
been  found  which  permit  an  accurate  determination  of  its  character. 
The  distal  end  is  not  compressed  as  it  is  higher  up,  but  is  round  in  cross- 
section  and  bears  a  facet  for  the  cuneiform.  A  rudiment  of  the  fifth  meta- 
carpal persists.  All  the  metacarpals  and  their  phalanges  are  somewhat 
shorter  and  less  massive  than  the  metatarsals  and  the  phalanges  of  the 
hind  foot.  The  pelvis  is  thoroughly  equine  and  yet  differs  in  many 
minor  characters  from  that  of  the  horse.  It  is  narrower  in  proportion  to 
its  length  than  that  of  the  latter.  The  ilia  expand  less  abruptly,  the  crest 
is  narrower  and  more  elongate  proportionately,  and  the  ischia  do  not 
bend  upward  posteriorly  as  in  the  horse,  but  are  in  a  straight  line 
with  the  long  axis  of  the  ilia.  The  obturator  foramen  is  more  elongate 
and  narrower  transversely,  and  the  pelvic  outlet  is  higher  and  narrower 
proportionately  than  in  the  modern  equine. 

The  fibula  was  complete  in  M.  bairdi  ;  was  very  much  reduced  in  size 
and  was  coossified  Avith  the  tibia.  The  proximal  end  is  quite  small,  the 
shaft  is  filiform,  while  the  distal  end  alone  is  quite  large  and  forms  the 
external  malleolus  articulating  with  the  astragalus,  and  in  extreme  ex- 
tension of  the  foot  also  with  the  calcaneum.  The  fibula  remains  com- 
plete until  John  Day  times,  for  in  Mesohippus  (Anchitherium)  prmstans 
Cope  from  this  formation  it  is  retained  in  its  entirety. 

The  hind  limbs  are  much  longer  than  the  fore  limbs,  more  so  propor- 
tionately than  in  the  horse,  so  that  the  rump  must  have  been  much  ele- 
vated above  the  withers  if  the  different  elements  of  the  limb  were  not 
very  much  more  flexed  on  each  other  than  would  seem  justifiable,  judg- 
ing from  recent  animals.  Many  of  the  White  River  animals  had  a  curved 
arched  back  instead  of  a  straight  back  as  in  the  horse,  e.  g.,  Hya>nodon, 
Leptomeryx,  etc.  This  is  shown  by  the  character  of  the  centra  of  the  ver- 
tebrae. The  great  individual  variations  met  with  in  J/,  bairdi  have  been 
noticed  by  every  investigator  who  has  studied  a  series  of  specimens  of 
this  species.  These  variations  are  principally  in  the  limbs  and  teeth. 
Some  of  these  have  already  been  noted.  In  several  individuals  the 
three  cuneiforms  of  tarsus  are  all  coossified  into  a  single  compound 
cuneiform.     Usually  the  ento-  and  meso-cuneiforms  are  united. 

There  is  usually  a  moderately  large  contact  of  metatarsal  iii  with 
the  cuboid,  this  latter  usually  extending  below  the  level  of  the  ecto- 
cuneiform,  so  that  all  contact  of  metatarsal  iii  with  cuboid  is  lateral. 
In  some  specimens  there  is  a  slight  extension  outwardly  of  the  proximal 


Farr.]  164  [May  15, 

end  of  M.  iii  and  the  cuboid  is  slightly  shorter,  so  that  it  articulates 
with  the  distal  end  of  cuboid  instead  of  being  confined  to  mere  lateral 
contact.  The  antero-internal  angle  of  cuboid  is  accordingly  somewhat 
modified  in  shape  to  correspond  with  the  changed  outline  of  metapodial 
iii.  This  is  a  tendency  in  the  direction  of  M.  intermedins  of  the  Pro- 
toceras  beds,  and  a  foreshadowing  of  the  condition  in  the  modern  horse 
which  has  such  a  large  facet  on  the  cuboid  for  the  widely  expanded  proxi- 
mal end  of  metatarsal  iii.  Between  this  condition  and  that  where  thei'e 
is  only  lateral  contact  with  the  cuboid,  we  find  all  the  intermediate 
stages.  Again,  there  is  a  great  deal  of  variation  in  the  relative  propor- 
tions of  the  lateral  digits  to  each  other,  and  in  the  relation  they  bear  to  the 
median  digit.  Sometimes  the  lateral  digits  are  not  much  reduced  and 
are  subequal  in  size,  while  again  we  find  the  lateral  digits  very  much 
reduced,  and  Mt.  iv,  at  least  proximally,  is  usually  larger  than  Mt.  ii. 

In  M.  hairdi  usually  there  is  no  confluence  of  posterior  transverse 
crest  with  the  oufer  wall  of  tooth,  usually  separated  from  it  by  a  large 
interval,  but  occasionally  we  get  an  individual  in  which  there  is  actual 
confluence,  and  we  get  all  stages  intermediate  between  these  two  ex- 
tremes. We  get  individuals  where  the  interval  between  outer  end  of 
transverse  crest  and  outer  wall  is  less,  and,  again,  others  in  which  there 
is  a  small  process  jutting  inwai-d  from  the  point  of  union  of  outer  lobes, 
toward  the  transverse  crest,  these  separated  by  a  very  small  interval, 
and  then  we  get  complete  confluence.  These  highly  specialized  forms 
were,  of  course,  not  ancestral,  but  were  prematurely  modernized  and 
left  no  descendants.  However,  these  individuals  most  specialized  occur 
highest  up  in  the  beds,  showing  that  these  variations  are  attempts  in  the 
way  of  evolution. 

Mesohippus  Copei. 

This  is  a  new  species  of  horse  from  the  White  River,  which  has  just 
been  described  by  Osborn  and  Wortman.*  In  their  description  of  the 
type  no  specific  characters  other  than  those  of  size  are  given,  by  which 
it  may  be  distinguished  from  the  two  other  species  from  this  horizon. 
This  species  was  founded  upon  a  complete  half  of  a  pelvis,  femur,  tibia 
and  part  of  a  hind  foot,  together  with  a  median  metatarsal  and  one  lat- 
eral metatarsal  of  another  individual,  a  collateral  type.  "These  re- 
mains indicate  an  animal  of  much  larger  size  than  those  of  M.  interme- 
dius,  and,  so  far  as  we  know,  is  the  largest  horse  of  the  White  River 
epoch,  even  larger  than  Mesohippus  (Anchitherium)  prastans  of  the 
John  Day."  The  species  is  undoubtedly  well  founded,  but  the  material 
in  the  Am.  Museum  did  not  permit  the  establishment  of  good  specific 
characters.  I  have  studied  carefully  the  material  upon  which  the  species 
is  founded  and  have  been  able  to  refer  some  material  in  the  Princeton 
Collection  to  this  species.     This  material  consists  of  the  distal  end  of  a 

*  Bull  Am.  Mm.,  Vol.  vii,  pp.  352-35S. 


1896.]  ibO  [Farr. 

femur,  tibia  and  almost  complete  hind  foot,  and  enables  me  to  give  some 
further  characters  of  the  species.  M.  copei  differs  from  M.  hairdi  in  the 
following  respects  :  (1)  The  lateral  metapodials  curve  outwardly  quite 
sharply  distally  and  the  toes  were  thus  more  spreading  than  in  M.  bairdi 
(see  Fig.  5).  (2)  The  meso-cuneiform  is  proportionately  less  deep  than 
the  ecto-cuneiform  than  in  M.  bairdi.  (3)  The  carina  or  median  keel  of 
the  distal  end  of  metatarsal  iii,  which  in  the  smaller  species  is  almost 
entirely  confined  to  the  plantar  surface  of  the  bone,  in  M.  copei  extends 
far  up  in  the  dorsal  surftice  of  the  distal  end  of  the  bone.  (4)  The  lat- 
eral metapodials  are  comparatively  shorter  than  the  median  metapodial, 
so  much  so  that  the  ungual  phalanges  could  scarcely  have  been  func- 
tional at  all,  and  this  form  had  progressed  farther  toward  monodactylism 
than  any  other  known  form  from  the  White  River.  (5)  The  combined 
depth  of  the  navicular  and  ecto-cuneiform  was  greater  than  in  M.  bairdi, 
and  greatly  exceeded  that  of  M.  interniedius.  (6)  The  cuboid  did  not 
extend  below  the  level  of  the  ecto-cuneiform.  Metatarsal  iii  was  borne 
by  the  latter  alone  and  did  not  extend  over  on  the  cuboid,  so  that  ante- 
riorly there  is  no  contact  of  these  two  bones  either  lateral  or  distal  as  in 
both  the  other  species. 

The  tibia  is  about  one  and  one-half  times  as  long  as  that  of  M.  bairdi, 
and  is  proportionately  much  stouter. 

The  shaft  is  very  long,  even  longer  than  that  of  the  John  Day  species, 
but  is  more  slender,  and  seen  from  the  side  it  presents  the  characteristic 
sigmoid  curve.  Tlie  cnemial  crest  is  very  high,  curves  slightlj^  outward 
and  has  the  usual  tendinal  sulcus  on  its  outer  border.  It  extends  farther 
down  on  the  shaft  than  in  M.  bairdi.  The  proximal  surface  is  very  much 
more  rugose  than  in  the  latter.  The  femoral  facets  slope  downward  and 
backward  at  quite  an  angle.  The  outer  facet  is  convex  antero-pos- 
teriorly  and  concave  transversely.  The  inner  facet  is  concave  antero- 
posteriorly  and  convex  transversely.  The  distal  end  of  tibia  is  turned 
slightly  outward.  The  distal  end  of  tibia  and  fibula  together  are  pro- 
portionately wider  than  those  of  M.  bairdi.  The  facets  for  the  trochlear 
surface  of  astragalus  are  deeply  incised,  are  oblique  in  position  and  are 
separated  by  a  high  intertrochlear  ridge. 

The  proximal  end  of  the  fibula  is  not  presei'ved,  but  the  very  large 
distal  end  and  a  portion  of  the  shaft  persists.  Rugosities  on  the  outer 
border  of  tibia  indicate  that  it  was  complete  and  closely  applied  to  the 
latter.  The  portion  of  the  shaft  preserved  is  very  much  reduced.  The 
expanded  distal  end  forms  the  external  malleolus  and  bears  the  two 
usual  facets. 

The  tarsus  presents  striking  differences  from  that  of  M.  bairdi,  and 
can  best  be  described  by  instituting  a  comparison  between  it  and  the 
latter. 

The  calcaneum  is  stouter  and  more  massive,  but  has  about  the  same 
relative  proportions  as  in  M.  bairdi.  The  tuber  calcis  is  large  and  rugose 
for  the  insertion  of  the  tendo  Achillis.     The  tuberosity  is  quite   high 


Farr.] 


166 


[May  15, 


with  its  inferior  border  slightly  convex.  The  upper  border  is  broken 
away.  The  tuberosity  is  much  thicker  and  more  mas- 
Fig.  5.  sive  than  in  the  smaller  species,  where  all  the  bones 
are  gracefully  shaped.  The  sustentaculum  is  very 
strongly  developed  and  bears  a  large  facet  for  the  as- 
tragalus, which  facet  is  elongately  oval  in  shape.  The 
crest  formed  by  the  superior  or  ectal  astragalar  facet 
is  broken  off  so  that  its  character  cannot  be  deter- 
mined. There  is  a  slight  prolongation  of  this  facet  an- 
tero-externally  which  is  somewhat  more  emphasized 
than  in  the  smaller  species.  The  inferior  facet  is 
near  the  distal  end,  and  is  the  smallest  of  all  the 
facets  of  calcaneum,  and  does  not  extend  far  back 
from  the  distal  end — elongate  in  shape.  The  facet  for 
the  cuboid  is  large,  occupying  all  the  distal  end  of  the 
bone  which  is  more  obliquely  truncated  than  usi;al. 
The  shape  is  triangular  with  the  apex  towards  the 
sustentaculum. 

The  astragalus  is  merely  an  enlarged  copy  of  that 
of  M.  bairdi  with  some  differences  of  detail.  It  is  pro- 
portionately broader.  The  trochlea  is  more  widely 
open  and  the  condyles  are  higher  and  thicker.  The 
neck  is  of  about  the  same  relative  proportions  as  in 
M.  bairdi.  The  internal  condyle  as  usual  is  the 
longer  of  the  two  and  anteriorly  slightly  overhangs 
the  navicular  facet  while  in  the  smaller  species  it  does 
not  quite  reach  it.  The  outer  condyle  is  very  much 
shorter  than:  the  inner  and  is  separated  from  the  navi- 
cular facet  by  quite  an  interval. 

The  navicular  is  a  flat  bone,  is  wide  transversely 
and  seems  proportionately  higher  than  in  M.  bairdi. 
articular  surface  is  strongly  concave  antero-poste- 
riorly  for  the  corresponding  surface  of  astragalus.  Posteriorly  there 
are  two  elevations  on  the  inner  and  outer  borders  respectively,  be- 
tween which  is  a  wide  and  shallow  depression  for  the  projection  on 
the  inferior  margin  of  distal  surface  of  the  astragalus.  The  exter- 
nal margin  of  this  latter  projects  strongly  downward,  extending 
around  the  outer  edge  of  navicular.  These  two  characters  make 
a  very  close  interlocking  joint  so  that  there  is  scarcely  any  direct  lateral 
movement  possible.  This  interlocking  is  not  quite  so  complete,  how- 
ever, as  in  M.  bairdi,  as  in  this  latter  the  external  margin  of  inferior  sur- 
face of  astragalus  extends  farther  down  on  outside  border  of  navicular. 
This  outside  projecting  border  is  in  the  form  of  a  crest  which  is  placed 
obliquel}"  on  bono  and  limits  the  direction  of  the  movement  of  the  two 
bones  taking  part  in  this  articulation  on  each  other  to  an  obliciue  motion. 
The  distal  surface  of  bone  presents  a  large  triangular  facet  for  ecto-cunci- 


Left  Foot  of 
M.  CoPEi,  1^. 

The    proximal 


1896.]  1^<  [Farr. 

form.  Coalescing  with  apex  of  above  is  a  facet  extending  up  on  posterior 
border  of  bone,  which  articulates  with  cuboid.  On  the  proximal  surff),ce 
there  is  a  small  facet  on  the  antero-external  corner  of  bone,  which  articu- 
lates with  the  calcaneum  by  a  small  facet  just  above  the  inferior  astraga- 
lar  facet  and  which  seems  to  be  a  part  of  the  latter,  but  on  close  exami- 
nation proves  to  be  a  distinct  facet.  In  31.  bairdi  the  navicular  just 
touches  the  calcaneum,  but  does  not  have  such  distinctly  marked  facets. 
This  character  is  seen  in  some  individuals,  but  in  all  observed  specimens 
the  contact  is  smaller. 

The  ecto-cuneiform  is  high  and  massive,  the  breadth  being  twice  the 
height.  The  proximal  facet  for  navicular  is  concave,  both  antero-pos- 
teriorly  and  transversely.  The  inferior  (or  distal)  focet  is  concave  in 
both  these  directions.  On  the  external  side  it  abuts  against  the  cuboid, 
and  this  latter  seems  to  have  been  just  equal  in  length  to  the  combined 
length  of  ecto-cuneiform  and  navicular.  It  bears  no  facet  either  lateral 
or  proximal  for  metatarsal  iv. 

The  coossified  ento-  and  meso-cuneiforms  show  an  emphasized  condition 
of  that  of  M.  bairdi,  in  that  the  tendency  of  the  distal  row  of  tarsal  bones 
to  form  a  closed  circle  is  more  marked  here.  The  portion  representing 
meso-cuneiform  bears  most  all  of  the  proximal  end  of  metatarsal  ii.  The 
ento-cuneiform  is  high  and  compressed  transversely  and  curves  strongly 
backward  and  around  towards  the  other  side  of  foot.  On  its  inferior 
surface  it  bears  a  facet  at  its  point  of  contact  with  metatarsal  ii. 

The  metatarsus  of  vJf.  5«iVfZ«i  exhibits  the  following  characters  :  (1)  The 
cuboid  which  bears  metatarsal  iv  extends  down  below  the  external 
cuneiform  which  bears  M.  iii.  (2)  The  meso-cuneiform  does  not  quite 
reach  to  level  of  the  ecto-cuneiform.  From  this  it  results  that  M.  iv 
does  not  quite  I'each  up  to  level  of  M.  iii,  while  M.  ii  reaches  above  the 
latter.  In  M.  copei,  M.  iv  extends  quite  up  to  the  level  of  M.  iii,  while 
the  meso-cuneiform  is  not  so  deep  proportionately  as  in  the  smaller 
species.  Metatarsal  iv  is  proximally  much  less  reduced  than  M.  ii,  but 
tapers  to  about  the  same  size  distally.  It  is  borne  entirely  by  cuboid. 
The  disproportion  in  size  of  the  proximal  ends  of  the  two  lateral  meta- 
podials  can  hardly  be  more  than  an  individual  character,  as  we  find  all 
degrees  of  difference  in  the  relative  sizes  of  the  two  lateral  digits  in  the 
smaller  species. 

In  some  specimens  the  two  lateral  digits  are  of  the  same  size,  in  others 
subequal  with  the  ivth  slightly  the  larger  and  in  others  this  digit  is 
very  much  larger  than  ii.  One  individual  exhibits  the  very  peculiar 
character  of  having  the  lateral  metapodials  of  the  same  size  on  one  foot, 
while  in  the  opposite  foot  the  fourth  metatarsal  is  much  larger  than  the 
second. 

Metatarsal  ii  is  slightly  less  rediiced  than  in  the  average  individual  of 
M.  bairdi.  Proximally  it  bears  a  large  concave  facet  for  the  meso-cunei- 
form and  posteriorly  there  is  a  small  facet  by  which  it  abuts  against  the 
inferior  retroverted  edge  of  the  ento-cuneiform.     This  latter  extends 


Farr.]  -»^bo  [May  15, 

both  above  and  below  the  meso-cnneiform  and  conditions  the  shape  of 
the  head  of  M.  ii,  about  one-half  of  the  proximal  surface  being  sup- 
ported by  the  meso-cuneiform.  Posterior  to  this  facet  the  proximal  sur- 
face slopes  abruptly  downward  and  presents  the  above-mentioned  facet. 
About  two-fifths  of  the  internal  surface  of  ecto-cuneiform  is  taken  up 
with  a  facet  for  metatarsal  ii,  which  in  M.  bairdi  extends  upward  pro- 
portionately less  on  the  ecto-cuneiform.  The  shaft  is  of  about  tlie  same 
dimensions  proportionately  as  in  M.  bairdi  and  was  closely  applied  to 
M.  iii  proximally,  but  both  the  lateral  metapodials  curve  outward  dis- 
tally.  The  distal  end  is  merely  an  enlarged  copy  of  that  of  the  smaller 
species,  is  high  and  compressed  and  the  median  keel  is  strongly  devel- 
oped. Metatarsal  iii  bears  about  the  same  relation  to  the  lateral  meta- 
tarsals in  size  as  in  M.  bairdi.  In  the  latter  we  have  a  distinct  facet  on 
M.  iii,  either  lateral  or  proximal  for  the  cuboid,  but  in  the  new  species 
M.  iii  does  not  touch  the  cuboid  and  the  only  facet  on  exterior  surface 
of  the  proximal  end  is  that  for  M.  iv.  It  is  borne  entirely  by  the  ecto- 
cuneiform  and  is  quite  large  in  proportion  to  the  size  of  the  lateral  digits 
and  supports  nearly  all  the  weight  and  receives  most  of  the  impacts  and 
strains  of  the  foot.  The  distal  end  is  somewhat  wider  than  the  proximal 
end.  M.  iii  is  quite  a  little  longer  than  the  lateral  metatarsals,  more 
so  than  in  M.  bairdi.  All  the  phalanges  are  slightly  more  massive  pro- 
portionately than  in  the  smaller  species. 

The  pelvis  in  the  Am.  Mus.  Collection  referred  to  M.  copei,  I  do  not 
regard  as  Mesohippus  at  all  because  it  is  too  much  specialized  in  its  own 
way  to  belong  to  a  White  River  equine.  It  difl'ers  very  much  from  that 
of  M.  bairdi  and  in  some  respects  is  more  specialized  than  that  of  the 
modern  horse.  If  the  reference  to  M.  coj^ei  is  correct,  we  have  in  this 
species  a  very  aberrant  side  line  of  the  horse  series.  The  pelvis  under 
discussion   differs   from   that   of  31.  bairdi  in   the   following  respects : 

(1)  The  ilium  expands  very  abruptly,  almost  directly  in  front  of  the  acet- 
abulum, while  in  M.  bairdi  it  expands  very  gradually  and  begins  its  ex- 
pansion a  long  way  in  front  of  the  acetabulum  (see  PI.  XIII  and  Fig.  4). 

(2)  The  angle  of  the  ilium  in  M.  bairdi  and  of  all  the  known  equines  is 
sharp,  but  in  this  specimen  it  is  very  much  rounded.  (3)  The  crest  is 
broad  and  stout  instead  of  being  narrow  and  elongate  as  in  M.  bairdi. 
(4)  The  border  between  angle  and  crest  is  very  much  less  concave  than 
in  M.  bairdi  and  the  horse.  (5)  The  border  of  bone  above  acetabulum 
is  drawn  out  into  a  sharp  crest  even  more  pronounced  than  in  the  recent 
horse.  (6)  The  acetabulum  is  round  as  in  Hyracodon,  not  elongate  as 
in  M.  bairdi  and  the  horse.  (7)  The  obturator  foramen  is  broader  in 
proportion  to  its  length  than  in  M.  bairdi.  (8)  The  ischia  turn  upward 
at  an  angle  posteriorly  almost  as  much  as  in  the  horse,  while  in  M.  bairdi 
the  ischium  is  in  a  straight  line  with  the  long  axis  of  the  ilium  and  does 
not  turn  up  posteriorly.  In  view  of  these  great  differences  I  cannot 
regard  the  reference  to  M.  copei  as  correct. 

In  the  American  Museum  there  are  a  series  of  lumbar  vertebrae  which 


1896.]  ^^^  [Farr. 

are  too  large  for  M.  intermedius,  and  their  provisional  reference  to 
J/,  copei  is  justifiable.  These  are  very  like  those  of  M.  hairdi,  but  much 
larger  and  more  massive.  The  provisional  reference  of  the  two  pre- 
molars described  with  the  type  is  also  justifiable,  as  they  are  too  large 
to  pertain  to  any  other  known  species  of  horse  from  the  White  River. 
Leaving  the  pelvis  out  as  questionable,  we  may  say  that  the  remains  in- 
dicate a  very  large  equine  agreeing  with  M.  bairdi  in  most  of  its  charac- 
ters and  yet  specialized  in  its  own  way  so  that  it  is  a  little  ofl:"  the  line  of 
equine  descent  though  most  probably  developed  from  M.  bairdi. 

Measurements  of  M.  copei. 

MM.         MM. 

Tibia 298        313 

Calcaneum,  length 83 

Calcaneum,  extreme  width 30 

Astragalus,  len gth 46  50 

Astragalus,  width  of  neck 31  37 

Height  of  navicular 11 

Height  of  ecto-cuneiform 11 

Length  of  M.  iii 177        189 

Femur,  distal  end  width 51 

Width  of  patellar  surface 29 

Extreme  length  of  first  phalanx  of  M.  iii 24 

"  "      second  "         "     11 

"  "       ungual  "         "     29 

Length  of  M.  iv 155 

Phalanx  1  of  M.  iv 14 

2  "  9 

3  "  22 

Mesohippus  intermedius  O.  and  W. 

M.  intermedius,  as  the  name  indicates,  stands  intermediate  between 
J/,  bairdi  of  the  Oreodon  beds  and  Mesohippus  {Anchitlierium)  proestans 
of  the  John  Day.  It  occurs  in  the  Protoceras  beds.  It  is  a  strange  and 
interesting  fact  that  M.  bairdi  continued  on  into  the  time  of  the  Protoceras 
beds  after  having  given  rise  to  the  two  species.*  A  careful  study  of  the 
principal  characters  of  M.  intermedius  brings  out  very  strongly  its  rela- 
tion to  the  preceding  and  succeeding  species.  In  all  these  points  it  is 
seen  to  stand  directly  intermediate  between  M.  bairdi  and  Mesohippus 
(Anchitherium)  priestans  of  the  John  Day.  In  the  light  of  present 
knowledge  there  can  be  no  doubt  that  M.  bairdi  is  the  direct  ancestor  of 
the  modern  horse,  and  by  the  study  of  the  individual  variations  of  the 

*  A  remarkable  instance  of  the  persistence  of  an  ancestral  type  is  seen  in  the  Loup 
Fork.  Here  Protohippus,  a  form  with  long-crowned,  cement-covered  molars,  represents 
the  main  line  of  equine  descent,  while  right  alongside  of  it  there  is  a  much  smaller  spe- 
cies of  M.  bairdi  type  which  Cope  has  called  Anchitherium  uUimuni.  This  form  has  short- 
crowned  molars,  without  cement. 


Fan-.]  I'^J  [May  15, 

former  we  can  trace  a  tendency  toward  the  establishment  of  the  M.  in- 
termedins type. 

The  skull  of  JSf.  iiitermedhis  is  much  more  equine  in  character  than 
that  of  J/,  baii'di.  It  presents  the  following  differences  which  may  be 
looked  upon  as  modernizations  :  (1)  Increase  in  length,  size  and  in  gen- 
eral proportions.  The  largest  skull  of  M.  bairdi  observed  measures  218 
mm.,  while  that  of  M.  intermedius  measures  280  mm.  (2)  The  upper 
incisors  are  all  pitted  (see  Fig.  6),  while  in  the  smaller  species  only  the 
p,     g  two  outer  pairs  have  the  enamel  invagination. 

(3)  The  facial  region  of  the  skull  is  more  elon- 
gate and  the  orbit  is  shifted  backward.  In  M. 
bairdi  the  anterior  border  of  orbit  is  over  M.  1  ; 
in  M.  intermedius  it  is  over  interval  between 
Ms.  2  and  3.  (4)  The  diastema  between  Pm.  1 
and  the  canine  is  proportionately  greater  in  the 
larger  species. 

The  canine  has  a  well -developed  cingulum  on 
its  internal  surface.     This  is  the  foreshadowing 

„  ^  of  the  cupping,  as  the  pit  in  an  incisor  tooth  is 

Superior  Incisors  and  i  i      »>  i 

Canine  of  M.  inter-  formed  by  the  cingulum,  which  rises  up  on  the 

MEDius,  1.  internal  border  of  the  tooth  to  enclose  the  de- 

pression. Teeth  have  been  observed  from  the  lower  Oreodon  l)eds  which 
have  a  strongly  developed  cingulum  anticipating  the  development  of  the 
pit.  (5)  The  occiput  is  slightly  more  overhanging  in  the  larger  species. 
(6)  The  aveolar  border  of  the  jaw  is  better  developed  and  higher  in  M.  in- 
termedins. This,  of  course,  is  correlated  with  larger  teeth,  with  longer 
roots.  (7)  The  postorbital  processes  are  better  developed,  more  nearly 
enclosing  the  orbit.  (8)  There  is  in  M.  intermedius  a  large  deep  ant- 
orbital  fossa  or  depression  occupjing  nearly  all  of  the  lateral  wall  of 
skull  and  extending  forward  almost  to  Pm.  1.  (9)  The  teeth  of  the 
molar  series  are  much  larger,  longer  and  more  specialized  than  those  of  if. 
bairdi.  These  differences,  which  have  been  given  by  Osborn  and  Wort- 
man  in  their  description,  are  :  (a)  "The  internal  cingulum  of  Pm.  1  is 
more  strongly  developed  and  a  distinct  basin  is  formed,  (b)  In  the  sec- 
ond upper  premolar,  the  parastyle  or  antero-external  buttress  is  consider- 
ably larger  than  in  M.  bairdi  and  gives  to  the  crown  an  incipient  trian- 
gular shape,  (c)  The  midrib  of  the  external  lobes  is  better  developed 
than  in  M.  bairdi,  and  the  postero-transverse  crest  is  more  nearlj^  con- 
fluenced  with  outer  wall  of  tooth." 

Length   of  Molar- Premolar  Series. 
M.bairdi....  73.5     M.  intermedins.,..  97     3/.  pra^sfans....  112.5 

Molar  Series. 
23  40  51 

Premolar  Series. 
43  53  61 


1896.]  lil  [Farr. 

The  Milk  Dentition. 

In  the  Princeton  Collection  there  is  a  skull  bearing  the  temporary  den- 
tition (No.  11168).  In  the  young  skull  the  anterior  border  of  the  orbit 
is  just  between  D.  4  and  M.  1,  so  that  as  growth  takes  place  the  orbit  is 
forced  to  retreat  by  the  elongation  of  the  facial  region  of  the  skull,  as  in 
the  adult  skull  the  anterior  border  of  the  orbit  is  over  the  interval  be- 
tween molars  2  and  3.  The  milk  teeth  agree  in  all  essential  points  with 
those  of  M.  bairdi. 

Measiu'ements  of  the  Superior  Milk  Teeth.        mm. 

Length  milk  series 57 

D.  1 10 

"  2 18 

"  3 16.5 

"  4 17 

The  lower  teeth  of  the  deciduous  set  agree  in  all  their  characters,  ex  ■ 
cept  size,  witli  tliose  oi  M.  bairdi. 

There  is  nothing  noteworthy  about  the  vertebrae  except  their  increase 
in  size  over  those  of  M.  bairdi.  Tlie  limbs  bear  the  same  general  pro- 
portions as  in  the  smaller  species.  The  scapula  is  higher  and  narrower 
proportionately  than  in  M.  bairdi.  All  the  limb  bones  are  characterized 
by  being  much  longer  than  in  the  smaller  species.  The  ulna  is  not  more 
reduced  distally  than  in  M.  bairdi,  and  is  distinct  from  the  radius  through- 
out. The  shaft  is  compressed  laterally  and  is  very  slender,  but  distallj' 
it  is  stouter  and  has  a  large  facet  for  the  cuneiform.  Proximally  the 
olecranon  is  more  massive  than  in  M.  bairdi.  The  radius  is  very  large 
and  is  fast  becoming  the  important  bone  of  forearm.  The  carpus  pre- 
sents no  important  difi'erences  from  that  of  the  smaller  species.  It  is 
still  high  and  narrow.  A  rudiment  of  the  fifth  metacarpal  still  persists, 
but  is  not  so  elongate  as  in  M.  bairdi,  but  is  shorter  and  stouter  and  on 
the  way  to  disappearing.  The  lateral  digits  are  usually  more  flattened 
than  in  the  smaller  species  but  are  not  more  reduced,  the  distal  ends 
being  even  more  massive  proportionately.  The  ungual  phalanges  of 
the  lateral  digits  are  long,  narrow  and  sharply  pointed  at  the  ends. 
That  of  metacarpal  iii  is  proportionately  wider  than  that  of  M.  bairdi. 

MM. 

Length  of  M.  iii 155 

M.  iv 138 

M.  ii 143 

The  ribs  are  characterized  by  their  length  and  extreme  slenderness, 
those  of  the  median  dorsal  region  being  especially  long,  not  much  flat- 
tened, being  almost  round  in  cross-section.  The  pelvis  presents  few 
characters  that  are  new.  The  ilia  expand  even  more  gradually  than  in 
M.  bairdi.     The  angle  rises  up  in  a  pointed  process.     The  crest  is  partly 


Fan-.]  l'^2  [May  15, 

broken  away  so  that  all  its  characters  cannot  be  determined.  The  bor- 
der of  bone  above  the  acetabulum  is  rounded  and  not  sharp.  The 
ischia  turn  upward  slightly  posteriorly  and  form  more  of  a  plate  poste- 
rior to  the  obturator  foramen  posteriorly  than  in  the  smaller  species.  The 
sacrum  has  five  vertebrae  entering  into  its  formation.  The  spines  of  the 
lumbars  are  still  very  high,  but  they  have  a  more  considerable  antero- 
posterior extent  proportionately  than  in  M.  hairdi.  The  femur  has  a 
massive  proximal  end,  the  great  trochanter  being  lower  and  more  mas- 
sive than  we  usually  see  it  in  Mesohippus,  but  this  may  in  part  be  due 
to  the  fact  that  our  skeleton  is  of  a  young  animal.*  The  tibia  of  M.  inter- 
medins is  somewhat  stouter  in  proportion  to  its  length  than  that  of  M. 
hairdi.  The  cnemial  crest  is  strong  and  well  developed.  As  usual, 
there  is  a  large  fossa  external  to  the  cnemial  crest.  The  fibula  is  still 
complete  and  is  distinct  from  tibia.  The  proximal  end  is  quite  small 
and  the  shaft  is  very  much  reduced,  while  the  distal  end  is  quite  large, 
forming  the  external  malleolus  to  articulate  with  astragalus  and  with 
calcaneum  in  extreme  extension.  Both  proximal  and  distal  ends,  as  well 
as  the  shaft,  are  closely  applied  to  the  tibia,  but  are  not  coossified  with 
it.  The  tarsus  of  M.  intermedins  is  more  modern  than  that  of  31.  bairdi 
in  that  the  tarsus  is  wider  and  lower,  which  is  a  step  in  the  direction  of 
the  modern  horse.  The  calcaneum  is  very  long,  the  tuber  proportion- 
ately longer  than  in  M.  bairdi,  and  is  quite  stout  with  an  expanded  free 
end.  The  cuboidal  facet  is  long  and  narrow,  almost  crescentic  in  shape 
and  extends  downward  and  inward  to  the  sustentaculum.  There  is 
quite  a  large  fibular  facet.  The  astragalus  is  broader  and  the  trochlea  is 
not  so  deeply  incised  as  in  M.  bairdi,  though  it  is  distinctly  equine  in 
pattern.  The  two  condyles  of  the  astragalus  are  very  unequal  in  size. 
The  inner  almost  always  overlaps  the  navicular  facet,  while  the  external 
is  separated  from  it  by  a  long  interval.  In  M.  bairdi  the  internal  con- 
dyle never  reaches  the  navicular  surftice.  The  navicular  is  much  flatter 
and  lower,  as  is  also  the  ecto-cuneiform,  than  in  31.  bairdi.  The  cuboid 
is  also  shortened,  just  equaling  the  height  of  the  two  contiguous  bones, 
metatarsal  iii  extends  over  on  cuboid. 

This  is  another  modernization.  There  is  a  distinct  facet  on  the  cal- 
caneum for  the  navicular.  There  is  a  much  more  complete  interlocking 
of  the  tarsal  bones  in  3f.  interinedius  than  in  any  other  White  River 
horse.  The  ento-cuneiform  as  usual  is  high,  extending  both  above  and 
below  the  meso-cuneiform  which  is  still  not  so  deep  as  the  ecto-cunei- 
form. On  its  posterior  surface  it  bears  a  distinct  facet  for  the  cuboid 
with  which  it  unites  in  forming  the  small  facet  for  M.  iv.  Metatarsal  iv 
is  usually  less  reduced  proximally  than  M.  ii,  but  tapers  to  about  the 
same  size  distally.  This  demonstrates  the  manner  in  which  the  reduc- 
tion of  digits  takes  place  in  the  family.  We  know  froni  31.  bairdi  that 
M.  i  first  disappeared  and  afterward  M.  v.  The  condition  in  31.  inter- 
medins indicates  that  M.  ii  would  next  become  rudimentary,  and  then 
M.  iv.     In  the  horse  where  the  lateral   metapodials  are  mere  splint 

*  This  may  also  account  for  the  fact  that  fibula  is  not  coossified  with  tibia. 


1896.] 


173 


[Farr. 


bones  and   closely  applied    to  M.  iii,  M.  iv  is  still   larger  than   M.  ii 
proximally. 

Tlie  inter-relationships  of  these  three  species  may  be  expressed  by  the 
following  diagram  : 


Protoceras  Beds. 


M.  bairdi.       M.  intermedins.  M.  copei. 


Orcodon  Beds.  /   M.  bairdi. 


M.  copei. 


Titanotherium  Beds.       /   M.  bairdi. 

The  phylogeny  of  the  horse  series  as  it  is  now  generally  understood 
may  be  given  as  follovrs  : 


Pliocene  to  Recent 


Loup  Fork 


Deep  River 


Equus 


John  Day 


White  River 


Uinta 


Bridffer 


Wasatch 


Puerco  Condylarthra 

J:  Represents  the  line  of  descent. 


Hippidium 


Protohippus  Hipparion 


Desmatippus  Anchitherium 


Mesohippus 


Mesohippus 


Epihippus 


Pachynolophus 


Hyracotherium  Palseotherium 


Protogonia 
Protogouodon 


Farr.]  1<4:  [May  15, 


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REPBINTED  SEPT.  7,  1896,  FROM  PROC.  AMER.   PHILOS.  SOC.  VOL.  XXXV. 


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